Today, off-highway trucks can have carrying capacities of 400 ton plus payloads. As such, these large trucks require very large truck bodies and in fact truck bodies on large 300 and 400 ton class capacity off-highway trucks often weigh in excess of 100,000 pounds or 50 tons. Periodically, it is necessary to remove these truck bodies from their truck chassis so that the bodies can be repaired or rebuilt and/or the truck chassis can be worked on. Repair or rebuilding of these truck bodies is often required, as the loading tools and the material being loaded/hauled in these truck bodies can over time cause substantial damage or abuse to the truck bodies. Also, some of the materials hauled in these truck bodies can be extremely hard and abrasive such as various hardrock ores, iron, copper, gold, etc. As such, these materials wear on the truck body floors and sides, thus requiring these truck bodies to periodically be lined/relined with abrasion resisting steel plate or like material and accordingly be rebuilt on occasion.
Additionally, often times it is necessary to remove a truck body for servicing of the truck chassis. Again, these truck bodies on large off-highway trucks are very large, approaching thirty feet wide and fifty feet in length; the truck bodies are in fact very large weldments.
Today the typical way to lift a truck body is to hook slings or cables to lifting eyes or lifting lugs on the sidewalls of the truck body and extend the slings or cables over the top of the truck body sidewalk to a point in the center of the truck body. To minimize the stresses on the truck body sidewalls, these slings or cables are attached to the truck body sidewalls at four points—two on either side of the truck body—and they are typically gathered to a central hook point. The cables are typically angled upwardly at a minimum angle of forty five (45) degrees (or greater) from horizontal, which results in the central hook point (lifting hook) on a thirty (30) foot wide truck body being at a height of half of the truck body width or fifteen feet above the truck body sidewalk.
In today's off-highway truck maintenance repair facilities 15 feet of overhead clearance or overhead crane lift clearance for lifting a truck body clear of the off-highway truck chassis when the slings are at this forty five (45) degree (or greater) angle is typically not available. Additionally, often times today where there is limited overhead clearance in the maintenance repair facility, two overhead bridge cranes have to be used. The two cranes are positioned on opposing sides of the truck body to minimize the amount of hook height needed to lift the truck body. But many times this still does not provide enough free overhead bridge crane hook height.
Additionally, over a truck body's life the truck body sidewalls may weaken particularly because of the hauled material abrasion and the repeated impact from the material being hauled in the truck body. Also the body sidewalls in large 300 and 400 ton ultra class payload trucks are often nine to ten feet deep. So when hooking to/over these body sidewalk, with slings or cables from each of the four corners of the body, which are collected at a central lifting point in the body center, extreme stresses are put on the sidewalls, pulling them inwardly. Users and operators of lifting rigs using this method are concerned with using the lifting points in the body sidewalls typically incorporated in the off-highway truck manufactures' standard truck bodies. The users and operators are concerned that over time the body sidewalls will weaken and not be able to withstand the compressive forces pulling inwardly on the sidewalls as a result of the slinging and hooking over the top of the body sidewalls from a lifting hook to attachment points on the body sidewalk.
In fact, many mines refuse to hook to body sides in lifting large truck bodies off their truck chassis. Rather, they weld lifting eyes to the truck body floor at some lesser distance apart than the overall truck body width. If the truck body width is thirty feet, these lifting eyes may be welded eighteen feet apart or nine feet from the center of the truck body. So when cables/slings are hooked to these lifting eyes and collected at a central point, an overhead bridge crane does not need nearly as much height under its hook to be able to lift the truck body.
Typically the overhead bridge cranes in truck service/maintenance facilities are of a fifty, sixty ton, or higher lifting capacity. Such an overhead bridge crane can lift a truck body where minimal free hook lifting height is available. Today, the options are either to hook to the original lifting points mounted in the truck body sidewalls, slinging over the sides of the truck body to a central point over the center of the truck body or, as many mines do, weld lifting eyes to the truck body floor and rely on the quality of those welds to lift a body. The welded lifting eyes need a much lower hook height such as what exists under the overhead bridge cranes in many off-highway service/maintenance facilities. However, the weld required to support the weight of the body is substantial. Repeated fastening of lifting points to the body floor by welding lifting points to the floor risks degrading the structural integrity of the floor, making the lifting of the body from the floor less desirable despite its clear mechanical advantage compared to lifting from the sidewalls.
Furthermore, if the existing lifting points on the truck body sidewalls are used and the cables or slings are passed over the truck body sidewalls, workers often have to climb up to the outside of the truck body sidewall, which is often in excess of 20 feet off the ground, to attach the lifting sling or cable hook to the truck body sidewall lifting points.